(a) Field of the Invention
The present invention relates to semiconductor optical amplifiers used in optical communication systems or optical switchboard systems.
(b) Description of Related Art
In optical fiber communication systems, optical amplifiers play an important role in attaining a longer transmission distance. Especially, technical improvements in optical fiber amplifiers having a core doped with erbium have developed remarkably, which has greatly furthered practical use of optical fiber communication together with increased output power and reliability of light exciter. Compared with optical fiber amplifiers, semiconductor optical amplifiers are smaller, consume less power and can be formed in an array more easily. Accordingly, they are especially important for applications such as loss compensation for optical switches used in multi-channel optical transmission systems or optical switchboard systems.
Such a semiconductor optical amplifier should have a large gain and a high saturation optical output power while the gain should have a low dependence on the plane of polarization (hereinafter referred to as a "polarization sensitivity" or "polarization plane dependence"). In an ordinary semiconductor optical amplifier having an embedded active layer, since the cross-sectional area of the active layer perpendicular to the propagation direction of a signal light is constant, the intensity of the signal light becomes strong in the vicinity of the emitting end for the signal light (hereinafter referred to as "light-emitting end") due to amplification of the signal light, resulting in an increased consumption of carriers. This is apt to induce a saturation of gain.
To solve this problem, an improved semiconductor optical amplifier is proposed in which the cross-section of the active layer perpendicular to the propagation direction of light is made larger from the receiving end for the signal light (hereinafter referred to as a "light-receiving end") toward a light-emitting end has been proposed, as described in JP-A-89-268084.
In the semiconductor optical amplifier described in the publication as mentioned above, however, the saturation value of optical output power is lowered because the mode cross-sectional area of light becomes smaller when the width and thickness of tile active layer at the light-emitting end are increased. Moreover, the polarization plane dependence of gain increases when only the width of the active layer is increased. Accordingly, it has been difficult to obtain a semiconductor optical amplifier which attains a large gain, a low polarization plane dependence of gain and a large saturation value of optical output power simultaneously.